The invention relates in general to semiconductors and in particular to a method for securely bonding together first and second functional layers of semiconductors. Multilayer systems comprise at least two mutually adhering layers and serve, for example, as sensors for detecting substances such as gases. Those layers that detect the substance are typically referred to as functional layers.
A disadvantage of such functional layers is that they may not adhere together with sufficiently high strength. For this reason, an additional intermediate layer yielding a greater adhesive action may be inserted between the two functional layers. However, the intermediate layer may have the disadvantage in that this layer may diminish the functionality of the functional layers.
In the case of a hydrogen sensor, for example, adequate measuring accuracy requires the cleanest possible interface between one functional layer made of, for example, palladium and another functional layer made of, for example, silicon nitrite. However, these two materials comprising the functional layers generally do not adhere together relatively well. As a result, disposed between these two functional layers is usually an intermediate layer which adheres relatively well to both functional layers. Nickel may be used as the intermediate layer to adhere the two functional layers of the hydrogen sensor together. However, the use of nickel typically impairs the functionality of a hydrogen sensor to some degree.
Thus, in general, the functional layers should adhere strongly together, yet their functionality should not be impaired.
As a result of this tradeoff, the method described in German patent DE 42 40 996 C1 is not generally applicable. Therein, two layers are bonded using a layer of adhesive applied therebetween, which is passed through in pointwise fashion by islands of fast-setting hot-melt adhesive. These adhesive islands fix the layers with respect to one another until the layer of adhesive applied in a large-area manner takes effect. With macroscopically thick layers of adhesive, this method is well-suited to the bonding of circuit arrangements to a circuit board, but less suitable to the use of functional layers, because the thick layers of adhesive may impair the functionality of the components. Further, this method presupposes that a material is known with which a firm bond can be produced between the various layers.
German patent application DE 28 21 303 B1 describes a method for bonding an insulating substrate to a metal coating, where the insulating substrate has a certain chemical composition. The chemical composition makes possible a selective etching operation by which depressions are made in the insulating substrate, thereby increasing its surface area. Afterward, a metal coating is deposited on the substrate in a currentless manner. The metal coating enters into a stronger bond with the substrate because of the increased surface area of the substrate. This method has the disadvantage that it is utilized with only those layer systems in which the requisite adhesive force can be achieved through an increase in the adhesion area. If the materials of the two layers do not generally adhere to one another, or if the requisite adhesive force is not attained even after the increase in the surface area, then this method does not achieve the desired success.
Similarly, the method described in German patent application DE 197 18 177 A1 utilizes an increase in the surface area of a substrate layer to improve the adhesion between this substrate and a second layer to be applied. Therein the substrate surface area is increased by first applying opaque particles to the substrate surface, which masks the substrate surface. Next, material is stripped off in the unmasked regions of the substrate surface, for example by a laser. This has the effect of roughening the substrate surface. After the masked particles are removed, the second layer is applied to the substrate surface and, because of the increased adhesion area, a stronger bond between the two layers results. This method has the advantage that when the material-stripping light is obliquely incident, webs remain on the substrate surface, which webs narrow toward the substrate surface. If the second layer is subsequently applied to the substrate, a type of keying interlocking results between the substrate and the second layer. This results in enhanced adhesion and, in particular, bonding between two layers that may otherwise not adhere to one another. Due to features of the method, the webs serving as anchoring elements may be made of the same material as the substrate. As a consequence, if the layer to be applied adheres poorly or not at all to the substrate, it may be necessary to provide a relatively high number of wedge-shaped webs or anchoring elements to ensure adequate adhesion between the layers to be bonded. If the webs are not fashioned in wedge shape, a strong adhesive bonding of the layers may not be possible.
What is needed is a method for producing a relatively strong bond between two functional layers of a semiconductor while at the same time allowing for a relatively high degree of functionality of the layers.